There are three common states of matter, solid, liquid, and gas. Liquid crystal (LC) is a fourth state that certain kinds of matter can enter into under the right conditions. Molecules in solids exhibit both positional and orientational order. The molecules are constrained to point only certain directions and to be only in certain positions with respect to each other. In liquids, the molecules do not have any positional or orientational order, the direction the molecules point and their positions are random.
The liquid crystal phase exists between the solid and the liquid phase. The molecules in liquid crystal may or may not possess any positional order, but they do possess a certain degree of orientational order. The molecules do not all orient the same direction all the time, they just tend to orient more in one direction over time than other directions. This direction is referred to as the director of the liquid crystal.
Most liquid crystal compounds exhibit more than one phase in the liquid crystalline state. The term mesophase is used to describe liquid crystal phases of materials. Different mesophases are formed by changing the amount of order in the sample, either by imposing order in only one or two dimensions, or by allowing the molecules to have a degree of translational motion. Typically phase transitions occur with changing temperature (thermotropic liquid crystals)
A mesogen is the fundamental unit of a liquid crystal, a molecule or small supramolecular assembly, that induces structural order in the fluid phases. Typically, a liquid crystalline molecule consists of a rigid moiety and one or more flexible parts. The rigid part (the core) is the major driver of orientational order, whereas the flexible parts (tails) induce fluidity in the liquid crystal. The optimum balance of the rigid and fluid parts is essential to forming liquid crystalline materials.
The tendency of liquid crystal molecules to point along the director leads to a condition known as anisotropy. Anisotropy means that the properties of a material depend on the direction in which they are measured. If the alignment is large, the material is very anisotropic. Similarly, if the alignment is small, the material is almost isotropic. The anisotropic nature of liquid crystals is responsible for their unique optical properties.
The nematic liquid crystal subphase is characterized by molecules that have no positional order but tend to point in the same direction along the director. The molecules point vertically but are arranged with no particular order.
The smectic state is another distinct mesophase of liquid crystal substances. In this mesophase, the molecules have one-dimensional positional order, forming a lamellar structure. Molecules in this phase show a degree of translational order not present in the nematic. In the smectic state, the molecules maintain the general orientational order of nematics, but also tend to align themselves in layers or planes. Motion is restricted to within these planes, and separate planes are observed to flow past each other. In some smectic mesophases, the molecules are affected by the various layers above and below them. Therefore, a small amount of three dimensional order is observed. The increased order means that the smectic state is more “solid-like” than the nematic. Many compounds are observed to form more than one type of smectic phase; as many as 12 of these variations have been identified.
Chirality is a property of a molecule. A molecule is either chiral or achiral. If a molecule is chiral, it means that it is non-superimposible upon its mirror image. A chiral molecule will interact with polarized light. The properties of the light will change after it has interacted with the chiral molecule. For example, in a given chiral smectic mesophase, the director makes a tilt angle with respect to the smectic layer. The director of the chiral smectic mesophase is not parallel or perpendicular to the layers, it rotates from one layer to the next.
Liquid crystals can be formed from many different kinds of compounds. Many of the liquid crystalline compounds are rod like, some are disc like and still others have a bent-core structure. Several liquid crystalline subphases exist for each of these different compounds. Rod shaped and disc shaped liquid crystalline molecules have been thoroughly studied. However, bent-core liquid crystalline molecules have only recently been studied using techniques from both condensed-matter physics and chemistry. The most commonly studied bent-core molecules are alkyl-tailed bis-Schiff base bent-core diesters, see FIG. 1. R is CnH2n+1 were n suitably ranges from 7 to 18.
One particular molecule of this family, the n-nonyl tailed version (NOBOW), is commonly used as a representative bent-core material in experiments. Most commonly, there are seven different liquid crystalline subphases observed in bent-core liquid crystalline molecules; B1, B2, B3, B4, B5, B6 and B7. Each subphase possesses its own unique physical properties. The characteristics of the subphases are due to multiple, competing forces at work within a given subphase. There is usually a competition between local preference and global ordering between chirality and layering, present in all chiral crystals and layered chiral liquid crystals. The chiral packing of molecules induces local molecular twist, but macroscopic ordering into layers expels twist, leaving the local organization strained. In fluid layered, smectic liquid crystals of rod-shaped molecules, this competition produces inhomogeneous phases in which molecular and layer twist coexist, the latter enabled by periodic arrays of twist grain boundaries or melted sheets. Systems of simple bent-core molecules having a particularly strong coupling between chirality and layering originate from the requirement to accommodate the size and shape of molecular sub-fragments in the presence of a robust tendency for layering. The bent-core molecules are achiral, but spontaneous polar order and chirality appear as broken symmetries, coupling to drive local Gaussian curvature, also known as saddle splay deformation, of the layers. This Gaussian curvature is a local solution that cannot fill space. This frustration leads to a spectacular hierarchical structure in which layering can appear only if twisted, doing so in the form of nanofilaments of twisted layers, see FIG. 4. The nanofilaments in turn collectively organize into a homochiral liquid crystalline array with coherent twist. Such macroscopic nanoporous assemblies of helically precessing layers are unanticipated solutions to the problem of obtaining coexisting layering and twist in a condensed phase.
As shown in FIG. 4A, NOBOW molecules form layers with negative curvature. The structure if polar along the ribbon axis, indicated by the vector P. FIG. 4B shows that twisted layers self-assemble into nanorods composed of 5-7 layers. The rods are self-limiting in size in two dimensions due to the free energy cost of deviating from the preferred curvature. FIG. 4C is a freeze fracture transmission electron photomicrograph showing nanorods with twist “in phase” in an LC of rods.
Different bent-core molecules are predisposed to forming different subphases. The B1 subphase occurs only in the shorter-tailed members of the bent-core molecules. NOBOW, having a nine carbon alkyl tail at both of its ends, exhibits only the B2, B3, and B4 phases. Although each of these B subphases consists of some packing structure dependent on the bent nature of the molecules, little is known about how to predict phase sequence based on chemical structure. Certain groups within the structure of the bent-core molecule are known to encourage specific behaviors, but small changes can have drastic effects on the mesogen behavior of the molecule. For example, simply changing the length of the tail can cause the disappearance of a phase, or change its transition temperatures.
The B4 phase is unique to bent-core molecules. It typically occurs towards the lower end of the liquid crystal temperature range for molecules that exhibit it. The B4 phase often, but not always, forms as a conglomerate consisting of large (typically hundreds of microns across) heterochiral domains, so although it has a helical structure it is not biased towards one handedness or another. When viewed through a crossed polarizer and analyzer its texture is blue (or occasionally grey) and almost featureless, but when polarizer and analyzer are decrossed in one direction half the domains darken and half brighten. If decrossed in the other direction, then the domains that were previously darkened are now brightened, and vice-versa. This behavior is consistent with a phase that is achiral overall, though it consists of a conglomerate of chiral domains. It therefore is not caused by molecular chirality but by macroscopic, phase-induced chirality. Materials reported to demonstrate the B4 phase include, without limitation, those reported in the attached BIBLIOGRAPHY. Some molecules that enter the B4 phase do not show chiral domains in the neat material.
B4 materials are characterized by twisted rods, or layer fragments with negative curvature. A generally known B4 “texture” ascertainable by transmission electron microscopy (“TEM”) is also diagnostic to persons of ordinary skill ion the art. By way of example, one skilled in the art may observe the texture of freeze-fracture TEM to recognize the B4 texture as reported in Hough et al., “Helical nanofilament phases,” Science 2009, 325, (5939), 456-460. Additional confirmation of the B4 phase may reside in the observation of twisted nano-rods grown from the neat LC, or from solution, and also in the observation of slow dynamics by solid state NMR.
Development of new materials capable of providing useful solar energy less expensively than coal is certainly one of the most challenging problems facing society. Solar energy is captured for useful electrical power using a photovoltaic (PV) cell, also known as a solar cell. PVs may be made from inorganic (IPV) or organic (OPV) materials. OPVs are one of several contenders in renewable energy technology potentially providing a way to convert solar energy into electricity using inexpensive organic materials and devices. By way of example, U.S. Pat. No. 8,357,849 describes organic the operation of organic photovoltaic devices in terms of relativde energy levels. A first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level.
In the context of organic materials, the terms “donor” and “acceptor” refer to the relative positions of the HOMO and LUMO energy levels of two contacting but different organic materials. This is in contrast to the use of these terms in the inorganic context, where “donor” and “acceptor” may refer to types of dopants that may be used to create inorganic n- and p-types layers, respectively. In the organic context, if the LUMO energy level of one material in contact with another is lower, then that material is an acceptor. Otherwise it is a donor. It is energetically favorable, in the absence of an external bias, for electrons at a donor-acceptor junction to move into the acceptor material, and for holes to move into the donor material. A significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. In the context of organic photosensitive devices, a layer including a material that conducts preferentially by electrons due to a high electron mobility may be referred to as an electron transport layer, or ETL. A layer including a material that conducts preferentially by holes due to a high hole mobility may be referred to as a hole transport layer, or HTL. Preferably, but not necessarily, an acceptor material is an ETL and a donor material is a HTL.
Conventional inorganic semiconductor PV cells employ a p-n junction to establish an internal field. Early organic thin film cells, such as reported by Tang, Appl. Phys Lett. 48, 183 (1986), contain a heterojunction analogous to that employed in a conventional inorganic PV cell. However, it is now recognized that in addition to the establishment of a p-n type junction, the energy level offset of the heterojunction also plays an important role. The energy level offset at the organic D-A heterojunction is believed to be important to the operation of organic PV devices due to the fundamental nature of the photogeneration process in organic materials. Upon optical excitation of an organic material, localized Frenkel or charge-transfer excitons are generated. For electrical detection or current generation to occur, the bound excitons must be dissociated into their constituent electrons and holes. Such a process can be induced by the built-in electric field, but the efficiency at the electric fields typically found in organic devices (F˜106 V/cm) is low. The most efficient exciton dissociation in organic materials occurs at a donor-acceptor (D-A) interface. At such an interface, the donor material with a low ionization potential forms a heterojunction with an acceptor material with a high electron affinity. Depending on the alignment of the energy levels of the donor and acceptor materials, the dissociation of the exciton can become energetically favorable at such an interface, leading to a free electron polaron in the acceptor material and a free hole polaron in the donor material.
Solar cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved efficiencies of 41% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. Present commercially available amorphous silicon cells have stabilized efficiencies between 4 and 8%. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs. These materials, if sufficiently cheap to manufacture, could compete with using polluting fossil fuels as a source of energy.
Currently the most commonly used PVs are made of inorganic materials. In inorganic materials, fine control of electronic properties is required to achieve the desired photovoltaic behavior. The material is doped to increase the number of negative (n-type) charge carriers, typically electrons, or positive (p-type) charge carriers, typically called “holes.” This is accomplished by replacing some fraction of the atoms with elements having less or more valence electrons, creating a deficit or surplus of electrons. The resulting p- and n-type semiconductors are then joined to form a p-n heterojunction that allows the generation of electricity when illuminated. As a photon strikes the material, an electron-hole pair is created. These will typically recombine within microseconds, but if this pair is close enough to the p-n heterojunction the two can separate from each other and generate current.
In semiconductors and insulators, electrons are confined to a number of bands of energy, and forbidden from other regions. The term “band gap” refers to the energy difference between the top of the valence band and the bottom of the conduction band. Electrons are able to jump from one band to another. However, in order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition. The required energy differs with different materials. Electrons can gain enough energy to jump to the conduction band by absorbing either a phonon (heat) or a photon (light).
Semiconductors have a small but nonzero band gap which behaves as an insulator at absolute zero but allows thermal excitation of electrons into its conduction band at temperatures which are below its melting point. In contrast, a material with a large band gap is an insulator. In conductors, the valence and conduction bands may overlap, so they may not have a band gap.
The conductivity of intrinsic semiconductors is strongly dependent on the band gap. The only available carriers for conduction are the electrons which have enough thermal energy to be excited across the band gap. For instance, the band gap of silicon is 1.1 eV, so it is incapable of absorbing any photons with less energy than this. However, if a higher-energy photon is absorbed by silicon, all energy over 1.1 eV is dissipated as heat. Therefore, PVs must be engineered to have the correct band gap.
OPV devices typically have relatively low quantum yield (the ratio of photons absorbed to carrier pairs generated, or electromagnetic radiation to electricity conversion efficiency), being on the order of 1% or less, but may reach up to about 8% in the most recent OPV devices. This is, in part, thought to be due to the second order nature of the intrinsic photoconductive process. That is, carrier generation requires exciton generation, diffusion and ionization.
An exciton is a bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. The exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge.
An exciton forms when a photon is absorbed by a semiconductor. This excites an electron from the valence band into the conduction band. In turn, this leaves behind a localized positively-charged hole. The electron in the conduction band is then attracted to this localized hole by the Coulomb force.
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. One difficulty with doping in OPV cells is that the integrity of the organic semiconductor materials are often chemically or morphologically disturbed, which can create exciton traps or quenching sites that block diffusion and/or cause recombination. This difficulty can be overcome by using OPVs that are made from LCs due to the fluid-like nature of the LCs and thus their ability to “repair” any such flaws in the semiconductor material through minor translations of the constituent molecules. The B-4 subphase is a liquid crystal of nanorods, each composed of crystalline layers. The rods have crystalline characteristics.
Efficient movement of exitons through an OPV is a necessary quality of any organic solar cell. One significant measurement of the ability of OPVs to propagate the movement of exitons is carrier mobility. Carrier mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. As opposed to free carrier concentrations, carrier mobility is determined in large part by intrinsic properties of the organic material such as crystal symmetry and periodicity. Appropriate symmetry and periodicity can produce higher quantum wavefunction overlap of highest occupied molecular orbital (HOMO) levels producing higher hole mobility, or similarly, higher overlap of lowest unoccupied molecular orbital (LUMO) levels to produce higher electron mobility. Moreover, the donor or acceptor nature of an organic semiconductor may be at odds with higher carrier mobility. How to pair two organic photoconductive materials to serve as a donor and an acceptor in a photovoltaic heterojunction based upon carrier mobilities and relative HOMO and LUMO levels is well known in the art, and is not addressed here.
For additional background explanation and description of organic photosensitive devices, including their general construction, characteristics, materials, and features, see U.S. Pat. Nos. 6,657,378, 6,580,027 and 6,352,777 which are hereby incorporated by reference to the same extent as though fully replicated herein.